Excellent. This is an example of the usefulness of Bayesian reasoning, and it can be generalized to any situation where you are trying to use an observation of the form 'it-hasn't-happened-yet' to update your estimate of it's rate of occurring.

So, paraphrasing what you said, I first choose a reasonable range of probabilities for something-happening and a very rough estimate of my probabilities for those probabilities over that range. (For example, I think my house would sell in between 1 out of 50 and 1 out of 10 showings, and the probability should increase linearly over that range with some slope.) Second, each observation that something-hasn't-happened should update my probabilities as you described.

This is very interesting to me, that I can do something with the 'information' I get after each showing without an offer, and these calculations give me something to do while I'm waiting. (Besides continuing to stage my house, which I continue to work on as well even though I suspect I am in the region of diminishing marginal returns for that.)